1. Field of the Invention
This invention relates to an image forming device which prints an image on a recording sheet by performing a reciprocating movement of a print head in a main scanning direction while transporting intermittently, in a sub-scanning direction, the recording sheet that is attracted to a charged transport belt by an electrostatic force.
2. Description of the Related Art
In a conventional image forming device, it is known that an image is printed on a recording sheet by performing a reciprocating movement of a print head in a main scanning direction while transporting intermittently, in a sub-scanning direction, the recording sheet that is attracted to a charged transport belt by an electrostatic force. The transport belt of this image forming device is formed of a material having a small thickness. If ink adheres to the transport belt, the electrostatic force in the ink-adhesion portion of the transport belt will be changed to the value that is insufficient for attracting the recording sheet. Consequently, the recording sheet may be lifted from the transport belt when transporting the recording sheet, and the print head may rub the surface of the recording sheet being lifted. In such a case, there is a possibility that a printed image may be blurred or the print head may be damaged.
In order to avoid this, some improvements are made in the conventional image forming device. In these improvements, the rear-end position of a recording sheet is detected and the result of the detection is used to prevent ink from being adhered to the transport belt.
For example, Japanese Laid-Open Patent Application No. 2007-216670 discloses an image forming device which is arranged to delete an image when the necessary condition for keeping ink from adhering to a transport belt is satisfied.
Japanese Laid-Open Patent Application No. 2007-160681 discloses an image forming device which is arranged to determine a rear-end position of a recording sheet based on both a rear-end position that is measured using a sensor sensing the rear-end position of the recording sheet on the transport belt, and a rear-end position that is computed using a theoretical amount of transport of the recording sheet on the transport belt.
In the image forming device of Japanese Laid-Open Patent Application No. 2007-216670, the rear-end position of the recording sheet is simply computed based on the positional relationship between the print head nozzle position and the sensor position. However, when the recording sheet is intermittently transported in the sub-scanning direction, the accuracy of detecting the rear-end position varies according to the transporting distance of the recording sheet during the intermittent transporting process. For this reason, it is difficult to detect the rear-end position accurately. In a certain case, the printing operation may be continuously performed even when the rear end position of the recording sheet is exceeded, which causes the problem of adhering ink to the transport belt.
The image forming device of Japanese Laid-Open Patent Application No. 2007-160681 is arranged so that, when a difference between the measured rear-end position obtained using the sensor and the computed rear-end position obtained using the theoretical amount of transport of the recording sheet is smaller than a threshold, the computed rear-end position obtained using the theoretical amount of transport of the recording sheet is finally selected in order to eliminate the fluctuations of the detection accuracy during the intermittent transporting process.
However, in a case in which the user erroneously sets up the size of the recording sheet with the wrong one, or in a case in which the user erroneously places the recording sheet in the wrong orientation of length and width of the recording sheet, the measured rear-end position obtained using the sensor has to be finally selected as being the rear-end position of the recording sheet.
Even in such cases, in the image forming device of Japanese Laid-Open Patent Application No. 2007-160681, the computed rear-end position obtained using the theoretical amount of transport of the recording sheet is finally selected if the difference is smaller than the threshold, which causes the problem of adhering ink to the transport belt.